Interpretive Summary: Assessment of feeding value for feeds and forages includes the measurement of performance by animals that have production potential. This requires large amounts of the feedstuff, is expensive and laborious, and is not practical for screening large numbers of feeds. Therefore, partitioning components of forage quality attributes have historically included measurements or estimates of intake and digestibility. Good relationships have been reported between intake of digestible dry matter and rate of gain on selected forages. However, while these few studies show the relationship exists, large databases with intake, digestibility and performance are either lacking or the relationships are quite variable. Also, determinations of intake and in vivo digestibility are also laborious and time consuming, requiring significant amounts of the feed. While in vitro methods are available for estimating digestibility on small quantities (~0.5 g), no such method exists for intake. Intake is more variable among animals than is digestibility due to a greater impact of individual animal differences. Feed chemistry, physical characteristics and near-infrared reflectance spectroscopy have been used with varying success to estimate intake, digestibility and performance. Databases with a large number of feeds over varying plant species, environmental conditions and locations worldwide are needed to enhance the ability to predict feeding value consistently.

Technical Abstract:
A sound theoretical definition for forage or feed quality is animal performance. This definition may be useful as a relative comparison among forages when given to growing or lactating animals. Voluntary intake and nutrient digestibility have been used to form indices of forage quality, and most feeding standards and models are based on the assumption that animal performance is related closely to intake of available nutrients. Due to variation in measurements of intake, digestibility, and animal performance, however, relationships used to develop prediction equations for animal performance from intake and digestibility are often less accurate than desired. Some of the causes for inaccurate predictions include nutrient imbalances, environmental constraints on the animals used for measurements, and individual animal differences. Variation in voluntary intake is greater than that for digestibility, and appears to be more important in assessment of forage quality. Yet intake is more difficult to determine in animal trials and to predict from characteristics of the forage. To be useful in livestock feeding, forage quality information must be available before feeding. Due to expense, labor, time, and amount of the feed required, animal trials are not suitable for screening large numbers of feeds or forages such as those from genetic improvement trials. Therefore, prediction of forage quality from feed attributes taken from small samples is necessary. Chemical composition, in vitro bioassays, and near-infrared reflectance spectroscopy have been used successfully to predict intake and digestibility of defined sample sets such as those from genetic improvement trials, but have been more difficult to implement on unknown or open populations such as producer samples. The challenge to progress in this area is obtaining accurate intake, digestibility and performance data on an adequate number of samples under standardized conditions so that a suitable database is available for development of either robust equations, or equations with sufficient specificity to discriminate among different forage and genetic types.